The present application relates to a sensor for chemical sensing or bio-sensing applications, and a method of forming the same. More particularly, the present application relates to an ion-sensitive field effect transistor (ISFET) having an increased passivation capacitance, Cp, and enhanced sensitivity.
Ion-sensitive field effect transistors (ISFETs) are being explored as a biosensor or a chemical sensor. ISFETs can be fabricated along with a standard metal oxide semiconductor field effect transistor (MOSFET) to make a ‘lab-on-chip’ in which the ISFETs are used as sensors and the MOSFETS are used for signal processing.
In prior art ISFETs, a passivation layer such as, for example, a layer of silicon nitride, is typically formed above the last metal layer of a back-end-of-the-line (BEOL) structure, and a micro-well is formed in a dielectric material that is disposed above the passivation layer. The passivation layer serves two proposes. Firstly, the passivation layer prevents ions such as, for example, sodium ions and/or potassium ions, from traveling into the underlying transistor. Secondly, the passivation layer serves as a sensing layer to absorb electric charges from the analyte-containing solution to the surface of the passivation layer. In such ISFETs, the amount of electrical charges is measured by the threshold voltage change of the underlying transistor, thus the analyte (bio or chemical) in the micro-well is sensed.
Although passivation is necessary, the presence of the passivation layer in the ISFET reduces the sensitivity of the underlying transistor because the passivation layer creates a capacitance (often referred to as a passivation capacitance, Cp) in series with the FET capacitance (CFET). Compared with a case without a passivation layer (i.e., the analyte directing contacting the FET), the sensitivity of the ISFET is reduced to a factor A, where A can be expressed as A=Cp/(Cp+CFET), wherein Cp and CFET are as defined above.
The passivation layer is typically much thicker than the gate dielectric of the FET. Therefore, Cp is much less than CFET. As a result, A is much less than 1. Therefore, there is a need to improve the sensitivity of an ISFET by increasing Cp.